Most DNA in a cell is packaged around a set of histone proteins in a coiled structure known as a nucleosome. Nucleosomes, in turn, are further coiled into a highly condensed structure that tightly compacts the DNA. This combination of DNA and protein packaging is generally referred to as chromatin. Chromatin has two forms: euchromatin, a loosely packaged form of chromatin in which the DNA is accessible to transcriptional machinery and is usually, but not always, transcriptionally active, and heterochromatin, a tightly packaged form in which the DNA is inaccessible to transcriptional machinery and is usually, but not always, transcriptionally silent.
The transition between euchromatin and heterochromatin is mainly controlled by three epigenetic events, DNA methylation, histone modification, and RNA interaction. These epigenetic events affect whether genomic DNA in a cell is in a loosely packaged, transcriptionally active form or a tightly packaged, transcriptionally silent form.